The present invention relates to a head for pulling monocrystals from a molten product bath. This bath is contained in a crucible and is kept at a temperature above the melting point of the product by appropriate heating means. It is then possible to obtain a solid crystal with the aid of a monocrystalline nucleus of the same product placed at the lower end of a vertically axed pulling spindle, in accordance with the so-called Czochralski method.
The pulling heads of this type are mainly used for the crystallization of materials for which it is possible to find conditions where the crystallized solid has the same composition as the liquid (congruent melting body) such as certain oxides and semiconductors of the silicon and germanium types.
In such machines, the monocrystalline nucleus is brought into contact with the liquid bath, whose temperature is adjusted in such a way that at the solid-liquid interface the conditions are such that at each instant the crystallized atom quantity is identical to the atom quantity returning in the melt. Growth is obtained by exerting a regular vertical translatory movement on the pulling spindle at a speed which is generally between 1 and 30 mm per hour. In order to check the shape of the interface, on the translatory movement is superimposed a rotary movement of the spindle about its axis at a speed which is generally between 0 and 100 revolutions per minute. The upwards displacement of the solid-liquid interface brings the latter into a colder zone, which has the effect of crystallizing the bath on the nucleus and thus making the monocrystal grow.
Due to the slowness of the vertical translatory movement applied to the pulling spindle during a crystalline growth, it is conventional practice to provide the monocrystal pulling heads with fast spindle translation means, in order to bring about the lowering of the nucleus to the vicinity of the molten bath.
In the present state of the art, the slow and fast translatory displacements of the spindle are controlled by two conventional electric motors acting on a ball circulation system via a clutch and reduction gear. In the same way, the spindle is rotated by a geared motor system. The use of motors and reduction gears is obviously very costly. Moreover, the presence of these reduction gears introduces clearances and vibrations, which are prejudicial to the regularity of the movement required during the pulling of the monocrystal. Moreover, the use of a ball circulation system produces impacts during the slow displacement of the pulling spindle. The quality of the monocrystals obtained is reduced, particularly due to the presence of dislocations in these monocrystals.
The necessity of obtaining monocrystals with a regular geometry has led machine manufacturers to develop diameter regulations. In existing machines, where said regulation takes place by means of a weight measurement, a suitably positioned transducer continuously records either the weight increase of the crystal, or the mass loss of the molten bath.
The crucible weighing method is simple but, for experimental reasons, involves the unnecessary weighing of the refractory required for the thermal insulation of the system. Moreover, the measurement is subject to error resulting from the levitation forces exerted on the crystal in the case of a high frequency heating.
The method used in the present invention is the weighing of the crystal during growth. This method does not suffer from the disadvantages referred to hereinbefore and involves the insertion of a weight transducer into the spindle. This transducer is necessarily rotated by the latter.
According to the prior art, in order to supply information to the fixed parts, return instructions to the rotary part and supply the rotating transducer with electric power, it is necessary to use relatively unreliable revolving contacts, which have a limited service life.